I. Technical Field
In general, the present disclosure relates to polymerization catalysts and methods of using such catalysts in the production of polyethylene.
II. Background
Metallocenes are considered attractive catalyst components for applications in the production of polymers. In particular, non-bridged zirconocenes have been identified as attractive low cost catalysts for use in polymerization processes where little to no long-chain branching is desired. When looking at the performance of these catalysts, the development of catalysts typically focuses on maximizing both the activity and the maximum potential molecular weight of the resultant polymer. Ansa-bridged metallocenes tend to increase the amount of long-chain branching in the resultant polymer. Finally, hafnocenes can also be used to generate polymers but the catalysts are significantly more expensive and the catalytic activity is low compared to the zirconocenes based catalysts. Thus, a need still exists to develop highly active catalysts which can generate high molecular weight polymers with minimal long-chain branching.
Furthermore, many of the metallocene catalysts require a high ratio of aluminum from MAO to the metallocene catalysts in order to activate the catalyst. In these catalytic systems, the ratio of MAO to catalyst can reach over 100 to 1 thus greatly increasing the cost of using the catalysts. Other catalytic activators, such as perfluorophenyl borates, are expensive to use compared to MAO. Given that the activator is required in order for polymerization to occur, the activator provides another area of control to fine tune the product of the polymerization. The development of polymerization catalyst which becomes active at a relatively low amount of MAO or other activator would decrease the cost of producing the polymers. Furthermore, the development of a polymerization catalyst which allows the polymer produced to vary based upon the amount of activator used could allow for a simpler mechanism to control the properties of the polymer produced. Such a development would improve the ability to control the synthesis at a more detailed level through a simple mechanism reducing the complexity of production.